Process for reducing moisture content of ammonium nitrate



May 5, 1959 Filed May 29, 1956 L. LUBIN ET AL B. PROCESS FOR REDUCING MOISTURE CONTENT OF AMMONIUM NITRATE 3 Sheets-Sheet 1 y 1959 B. L. LUBIN ET AL 2,884,999

PROCESS FOR REDUCING MOISTURE CONTENT OF AMMONIUM NITRATE Filed May 29, 1956 3 Sheets-Sheet '2 May 5, 19 59 B.'L. LUBIN ET AL 2,884,999 PROCESS FOR REDUCING MOISTURE CONTENT OF AMMONIUM NITRATE Fil-ed May 29, 1956 s Sheets-Sheet s INVENTOR m ii iiw l x las.

ATTOR N EY United PROCESQ FOR REDUCING MOISTURE CONTENT OF AMMONIUM NITRATE Application May 29, 1956, Serial No. 588,119

2 Claims. (Cl. 159-47) Our invention relates to a process for the production of substantially dry ammonium nitrate and more particularly it relates to a process for the-removal of moisture from molten ammonium nitrate.

In the past the preparation of dry ammonium nitrate was accomplished by very involved and expensive processes which, moreover, were quite hazardous. These processes generally consisted of a neutralizing step to produce an ammonium nitrate solution, followed by an evaporating step to remove sufficient water so that crystallization, flaking, graining, prilling, etc. could be accomplished. Most of the processes also involved a final drying step in order to reduce the moisture content of the ammonium nitrate. These operations were expensive, involving much equipment, and the expenditure of large amounts of power and evaporation energy. Also, they were usually carried out in separate steps in the form of relatively small batches and the operation scattered over a fairly wide area so as to reduce the hazard from explosion. Because of the tendency of ammonium nitrate to explode at or above its melting point, it had not previ ously been considered possible to prepare it on a commercial scale by the direct interaction of ammonia and nitric acid at elevated temperatures.

US. Patent 2,568,901 describes a process for producing molten ammonium nitrate by the direct interaction of ammonia and aqueous nitric acid in the vapor phase. The reaction takes place in a reaction zone which is free to drain at its lower end so that the zone is maintained substantially free from liquid reaction products, thus minimizing the hazard from molten ammonium nitrate. In this process, the reaction products are removed from the reaction zone in two phases, one of which is steam produced by the large amount of heat given off by the exothermic nature of the reaction, and the other of which is molten ammonium nitrate. In this process, as the combined reaction product issues from the reaction zone, any free Water present is in the form of a vapor and is separated as a steam phase. In the course of the reaction, however, some of the moisture introduced with theaqueous nitric acid becomes dissolved in the molten ant-- monium nitrate forming a solution of a gas in a liquid. and this bound water does not separate from the combined reaction products with the steam phase. This dissolved moisture then carried through the process and. must be removed by a drying operation, generally car-- ried out after formation of the solid ammonium nitrate by any of such processes such as, for example, flaking, beading, prilling, etc.

We have now discovered a process for reducing the: moisture content of the molten ammonium nitrate con taining such dissolved or .bound water. Our process:

atom

is convenient, it does not require large amounts of expensive equipment, and it is economical to operate. By employing our new process, we are able to reduce the moisture content of themolten ammonium nitrate in.

commercial size operations to a point below 0.5% and.

generally to a moisture content of 0.2%.

ing point of 316 F.).

e ce

According to our new process, we pass an inert gas through the molten ammonium nitrate containing dissolved moisture in a stripping zone serving to promote intimate contact between the inert gas and the molten ammonium nitrate. Such a stripping zone can be filled with inert packing materials, which serve to increase the surface contact area or it can contain plates with bubble caps, etc. In our new process, the inert gas and molten ammonium nitrate can be contacted either concurrently or countercurrently; however, we prefer the latter. By thus treating molten ammonium nitrate, we are able to strip dissolved moisture from the molten ammonium nitrate and to thus obtain a substantially anhydrous product.

In the accompanying drawing, Figure 1 is a vertical section of an apparatus in which our process can be conducted. The apparatus consists'of a circular separator 1, with an inlet 2 for introduction of mixed steam and molten ammonium nitrate and a line 3 for outlet of steam. A stripping zone 4 is attached to the lower end of the separator 1, the stripping zone containing inert packing materials 5 held in position by screens 6 and 7. An inert gas inlet line 8 and a molten ammonium nitrate exit line 9 are attached to the lower end of the stripping zone.

In the operation of the apparatus of Figure l, a mixture of molten ammonium nitrate and steam enters the separator 1 through the inlet line 2. Steam separates from the mixture and exits from the separator through line 3. Molten ammonium nitrate flows down to the stripping zone 4 where it is countercurrently contacted by inert gas admitted through line 8. The inert gas is withdrawn from the system through line 3 while substantially dry molten ammonium nitrate is withdrawn through line 9.

Figure 2 of the drawing shows the moisture content of the molten ammonium nitrate produced in applicants process using stripping gas of different Water content. Figure 3 shows an expanded version of the portion of the graphs shown in Figure 2 adjacent to the origin where the various curves come together.

Figure 4 is a vertical section of another apparatus in which our new process can be conducted. In this apparatus, the stripping zone is fitted with plates 11, carrying bubble caps 12 and risers 13 instead of containing inert packing materials as shown in Figure 1.

In carrying out our process, we are able to employ as the stripping medium, any inert gas which remains in the gaseous state throughout the range of temperatures at which the process is operated. Satisfactory inert gases which we have employed include air and nitrogen.

The temperature at which we carry out our process for removing moisture from molten ammonium nitrate ranges from 340 F. to about 550 F., the lower temperature limit insuring sufficient fluidity of the molten ammonium nitrate to permit ready flow of the material through the packed stripping zone (pure ammonium nitrate has a melting point of approximately 337 F. and ammonium nitrate containing 1.0% moisture has a melt- The essential feature of the lower temperature limit is fluidity of the ammonium nitrate. When the stripping operation is conducted under vacuum, which operation, of course, is included within the scope of our invention, we can employ temperatures below about 340 F. and still maintain the molten ammonium nitrate in a fluid state.

When temperatures above about 550 F. are employed in the process, excessive decomposition of ammonium nitrate takes place. We prefer to operate our process at a temperature between about 370 and 430 F. since decomposition of ammonium nitrate is minimized at these temperatures and essentially all free water can be removed as a vapor phase at these temperatures.

The temperature of the inert gas employed in the stripping operation prior to contact with the molten ammonium nitrate must not be such as to alter the temperature of the molten ammonium nitrate outside of the range given above. For example, when the stripping operation is being conducted at temperatures near the lower limit of the range given above, it would not be possible to use an extremely low temperature inert gas such as would lower the temperature of the molten ammonium nitrate to a point where fluidity of the molten material was impaired. Similarly, it would not be possible when operating the stripping operation at temperatures near the upper limit of the range given above to employ inert gas at a very high temperature such that the temperature of the molten ammonium nitrate is raised to the point where excessive decomposition takes place. Under the preferred conditions of operation employing a temperature between about 370 and 430 R, we have found it necessary to preheat the stripping gas to a temperature of about 300 F. in order to insure fluidity of the molten ammonium nitrate especially when employing the bubble cap stripper.

In connection with decomposition of ammonium nitrate when in the molten state, it is important to note that the time at which the molten material is held in that particular state is a contributing factor to the extent of decomposition which takes place. The longer the molten ammonium nitrate is held at the high temperature, the larger the amount of decomposition which takes place. For this reason, we prefer to execute our stripping operation in the minimum length of time, since by thus operating our process we are further able to avoid excessive decomposition losses,

In connection with minimization of decomposition through increasing the rate of the stripping operation and minimizing the time required to accomplish the stripping operation, we prefer to employ the maximum rate of molten ammonium nitrate flow possible depending upon the size of the particular stripping zone available.

In the operation of our process, we have found that there is no critical lower limit for the amount of inert gas per unit amount of molten ammonium nitrate which can be employed. We have found that any amount of inert gas when passed through a molten ammonium nitrate in a stripping zone will effect removal of some of the bound moisture. However, we prefer to employ as high a rate of inert gas as possible without reaching the point at which entrainment of ammonium nitrate in the flow of inert gas takes place. The point at which entrainment takes place is known as the flooding point. The flooding point depends upon the rate of ammonium nitrate flow, the type of stripping means employed, the diameter of the stripping zone itself, the type of packing if a packed stripper is employed, and the size and number of bubble caps if they are employed, in addition to the rate of flow of the inert gas. Inert gas rates high enough to cause the flooding point to be exceeded and entrainment to occur can be employed; however such operation would require an entrainment separator in the system to prevent excessive losses of ammonium nitrate.

An important consideration in the operation of our process lies in the moisture content of the inert gas employed in the stripping operation. This is particularly important when air is employed as the inert gas since in such case air from the atmosphere is preferably used and it contains varying amounts of moisture. bviously, if extremely wet inert gas is employed in the stripping operation, the molten ammonium nitrate, would takeup mois: ture from the inert gas rather than the gas take up moisture from the ammonium nitrate as desired. Thus the humidity of the inert gas employed in the stripping operaand ammonium nitrate moisture content the amount of moisture in the inert gas employed must be less than the equilibrium value or removal of moisture from the ammonium nitrate will not occur. If the amount of moisture in the inert gas is above the equilibrium value, the ammonium nitrate will remove moisture from the inert gas rather than vice versa as desired. In theaccompanying drawing, Figure 2 is a graph of equilibrium values for various temperatures throughout the operative range. These curves show the concentrations of moisture in both ammonium nitrate and inert gas at which equilibrium between the two is effected for various representative temperatures throughout the operating range i.e. 340-550 F. For any particular temperature and moisture content of ammonium nitrate, the equilibrium value for the moisture content of the inert gas can be read from the equilibrium curve and to obtain removal of moisture from the ammonium nitrate under these conditions, the moisture content of the inert gas must be less than the equilibrium value determined from the curve. Thus the inert gas must be relatively dry as compared to the molten ammonium nitrate i.e. the moisture content of the inert gas must be less than the equilibrium value for the particular operating conditions. Obviously, it is desirable to employ inert gas which is as dry as possible. However, the inert gas which we prefer to employ is air because of its availability from the atmosphere and moisture is always present in the atmosphere.

The following examples are offered to illustrate our invention; however, we do not intend to be limited to the specific amounts, procedures, ranges, etc. given therein but rather we intend to include within the scope of our invention as described in this specification and the attached claims, all equivalents obvious to those skilled in the art.

Example I In a reaction system for the production of 280 tons per day of molten ammonium nitrate, ammonia and aqueous nitric acid were admitted to a reaction zone filled with inert packing materials, in which zone ammonia and nitric acid reacted to form molten ammonium nitrate and steam, the reaction products issuing from the reactor in two phases passed into a separator at a temperature of about 440 F. The steam phase was withdrawn at the top of the separator, while the molten ammonium nitrate settled to the bottom and flowed into a stripping zone consisting of a cylindrical shell 6 feet high with an inside diameter of 2.5 feet, the shell being packed through 4 feet 10 /2 inches of its length with /4 inch Raschig rings. The temperature of the molten ammonium nitrate entering the stripping zone was about 425 F. Air, preheated to a temperature of about 350 F. was admitted at the base of the stripping zone at the rate of 0.6 standard cubic foot per pound of molten ammonium nitrate to be stripped. The molten ammonium nitrate entering the stripping zone had a moisture content of from 0.8 to 1.2%. The molten ammonium nitrate after passing through the packed stripping zone was permitted to drain from the stripping zone through an exit line. The molten ammonium nitrate withdrawn from the packed stripping zone had a temperature of 420 F. and a moisture of about 0.25% -by weight.

Example II Ammonia and nitric acid were continuously reacted to produce molten ammonium nitrate at a rate of 200 pounds per hour, the reaction being carried out in a reaction zone filled with inert packing materials and free to drain at its lower end so that the reaction product, upon exit from the reaction zone, separated into two phases,'one of which was steam and one of which was substantially molten ammonium nitrate containing 0.96% moisture. The molten ammonium nitrate was passed tion is very important. For any particular temperature into a stripping v zone consisting of a five-foot length of 3-inch stainless steel pipe packed with 15 inch Berl saddles throughout four feet of its length. Air, preheated to a temperature of 300 was introduced at the bottom of the stripping zone at the rate of 0.3 standard cubic foot of air per pound of molten ammonium nitrate to be stripped. The moisture content of the molten ammonium nitrate entering the stripping zone was 0.96% by weight while the moisture content of the molten ammonium nitrate leaving the stripping zone was 0.20% by weight.

Example 111 Molten ammonium nitrate was produced at the rate of 276 pounds per hour by reacting ammonia and nitric acid in the vapor phase in a reaction zone filled with inert packing materials, the reaction zone being free to drain at its lower end. The reaction products were removed from the lower end of the reaction zone in two phasesa steam phase and a molten ammonium nitrate phase. The molten ammonium nitrate phase having a moisture content of 1.72% and a temperature of 396 F. was passed to a stripping zone consisting of an 18-inch long section of three inch stainless steel pipe filled with A inch Raschig rings. Nitrogen was introduced at the bottom of the stripping zone at the rate of 0.4 standard cubic foot per pound of ammonium nitrate to be stripped. Molten ammonium nitrate, having a moisture content of 0.8% by weight, was removed from the bottom of the stripping zone.

Example IV Molten ammonium nitrate was produced in the reaction system described in Example III except that the stripping zone was packed with A inch Berl saddles. Molten ammonium nitrate at a temperature of 506 F. and containing about 0.9% moisture was passed to the stripping zone at a rate of 230 pounds per hour. Air at ambient temperatures was introduced at the bottom of the stripping zone at a rate of 0.5 standard cubic foot per pound of ammonium nitrate to be stripped. Molten ammonium nitrate, having a temperature of 420 F. and a moisture content of 0.22% by weight, was removed from the bottom of the stripping zone.

Example V In a reaction system for the production of 250 tons of ammonium nitrate per day, ammonia and aqueous nitric acid were reacted as described in Example I. Following removal of the steam phase, the molten ammonium nitrate at a temperature of about 400 F. was flowed into a stripping zone consisting of a cylindrical shell 7 feet 4.75 inches high with an inside diameter of 2 feet 11 inches. A total of seven plates were disposed in the shell, each plate carrying 13 four inch bubble caps, each having a slot area of 4.14 square inches. Air preheated to a temperature of about 300 F. was admitted at the base of the stripping zone at the rate of 0.72 standard cubic foot per pound of molten ammonium nitrate to be stripped. The molten ammonium nitrate entering the stripping zone had a moisture content of 0.8 to 1.2%. The molten ammonium nitrate leaving the stripping zone had a moisture content of about 0.3% by weight.

This application is a continuation-in-part of our copending application Serial No. 422,782, filed April 13, 1954, and now abandoned.

Now having disclosed our invention, what We claim is:

1. In the dehydration of molten ammonium nitrate separated from a mixture of molten ammonium nitrate and steam and containing small amounts of dissolved water which is ordinarily inseparable from the combined reaction products with the steam phase, the improvement comprising heating the molten ammonium nitrate to from about 340 F. to 550 F. to maintain a temperature such that the ammonium nitrate is fluid and the ammonium nitrate decomposition rate is substantially inhibited, passing said fluid ammonium nitrate through a stripping zone filled with inert packing materials, intimately contacting said ammonium nitrate with an inert medium, gaseous within the above-described range of temperatures, bubbled through said packed stripping zone, the rate of flow of the inert gas through the molten ammonium nitrate contained in the stripping zone being of the order of 0.3 to 0.72 standard cubic foot of gas per pound of molten ammonium nitrate being stripped and which allows maintenance of the aforesaid temperature range within the stripping zone, and precludes entrainment of substantial amounts of ammonium nitrate in the stripping medium, said stripping medium containing less water content than the equilibrium value between the water contained in the inert stripping me dium and the water contained in the ammonium nitrate at the aforesaid operative temperature range, and withdrawing the ammonium nitrate from the stripping zone in a molten state and with a substantially reduced amount of the aforesaid dissolved water.

2. The process of claim 1, wherein the inert gas is passed through the stripping zone countercurrent to the flow of ammonium nitrate.

References Cited in the file of this patent UNITED STATES PATENTS 1,141,266 Raschig June 1, 1915 1,292,948 Zeisberg Jan. 28, 1919 1,713,045 Jacobson May 14, 1929 1,883,211 Wilson Oct. 18, 1932 2,089,957 Harris et a1 Aug. 17, 1937 2,402,192 Williams et al June 18, 1946 2,643,180 Miller June 23, 1953 

1. IN THE DEHYDRATION OF MOLTEN AMMONIUM NITRATE SEPARATED FROM A MIXTURE OF MOLTEN AMMONIUM NITRATE AND STEAM AND CONTAINING SMALL AMOUNTS OF DISSOLVED WATER WHICH IS ORDINARILY INSEPARABLE FROM THE COMBINED REACTION PRODUCTS WITH THE STEAM PHASE, THE IMPROVEMENT COMPRISING HEATING THE MOLTEN AMMONIUM NITRATE TO FROM ABOUT 340* F. TO 550* F. TO MAINTAIN A TEMPERATURE SUCH THAT THE AMMONIUM NITRATE IS FLUID AND THE AMMONIUM NITRATE DECOMPOSITION RATE IS SUBSTANTIALLY INHIBITED, PASSING SAID FLUID AMMONIUM NITRATE THROUGH A STRIPPING ZONE FILLED WITH INERT PACKING MATERIALS, INTIMATELY CONTACTING SAID AMMONIUM NITRATE WITH AN INERT MEDIUM, GASEOUS WITHIN THE ABOVE-DESCRIBED RANGE OF TEMPERATURES, BUBBLED THROUGH SAID PACKED STRIPPING ZONE, THE RATE OF FLOW OF THE INERT GAS THROUGH THE MOLTEN AMMONIUM NITRATE CONTAINED IN THE STRIPPING ZONE BEING OF THE ORDER OF 0.3 TO 0.72 STANDARD CUBIC FOOT OF GAS PER POUND OF MOLTEN AMMONIUM NITRATE BEING STRIPPED AND WHICH ALLOWS MAINTENANCE OF THE AFORESAID TEMPERATURE RANGE WITHIN THE ATRIPPING ZONE, AND PRECLUDES ENTRAINMENT OF SUBSTANTIAL AMOUNTS OF AMMONIUM NITRATE IN THE STRIPPING MEDIUM, SAID STRIPPING MEDIUM CONTAINING LESS WATER CONTENT THAN THE EQUILIBRIUM VALUE BETWEEN THE WATER CONTAINED IN THE INERT STRIPPING MEDIUM AND THE WATER CONTAINED IN THE AMMONIUM NITRATE AT THE AFORESAID OPERATIVE TEMPERATURE RANGE, AND WITHDRAWING THE AMMONIUM NITRATE FROM THE STRIPPING ZONE IN A MOLTEN STATE AND WITH A SUBSTANTIALLY REDUCED AMOUNT OF THE AFORESAID DISSOLVED WATER. 